The adenylate cyclase responsiveness of a variety of transformed cell lines was found to be depressed compared to their parental counterparts, an effect which correlated with the presence of the gene for the viral transforming protein. The guanine nucleotide regulatory protein which mediates stimulation of adenylate cyclase (Ns) was present in identical quantities is both normal and transformed cells, ad determined by cholera toxin labeling. Additionally, the ability of Ns from normal and transformed cells to reconstitute hormone sensitivity of unresponsive S49 cyc- cells was identical ruling out a defect in this component as an explanation for the observed results. Several normal and transformed lines were screened for the presence of the guanine nucleotide regulatory protein which mediates inhibition of adenylate cyclase (Ni). Although normal rat kidney cells (NRK) do have Ni there was no difference in the Ni activity in normal and transformed NRK cells. Furthermore, Madin Darby canine kidney cells (MDCK) do not exibit functional Ni activity, ruling out differential inhibitory input to adenylate cyclase activity as an explanation for depressed adenylate cyclase activity observed in transformed cells. Fusion experiments suggest that the decreased responsiveness of transformed cells does not reflect a defect in the catalytic component of adenylate cyclase since depressed activity is observed when endogenous catalytic activity is inactivated in fused hybrids. An explanation for depressed adenylate cyclase responsiveness of transformed cells is being investigated by comparison of regulation of endogenous protein phosphorylation fo normal and transformed cell membranes.